A Control-Oriented Model for Trajectory-Based HCCI Combustion Control

Previously, the authors have proposed the concept of piston trajectory-based homogeneous charge compression ignition (HCCI) combustion control enabled by a free piston engine (FPE) and shown its benefits on both engine thermal efficiency and emissions by implementing various piston trajectories. In order to realize the HCCI trajectory-based combustion control in practical applications, a control-oriented model with sufficient chemical kinetics information has to be developed. In this paper, such a model is proposed and its performance, in terms of computational speed and model fidelity, is compared to three existing models: a simplified model using a one-step global reaction, a reduced-order model using Jones-Lindstedt mechanism, and a complex physics-based model including detailed chemical reaction mechanisms. A unique phase separation method is proposed to significantly reduce the computational time and guarantee the prediction accuracy simultaneously. In addition, the paper also shows that the high fidelity of the proposed model is sustained at multiple working conditions, including different air-fuel ratios (AFR), various compression ratios (CR), and distinct piston motion patterns between the two end positions. Finally, an example is presented showing how the control-oriented model enables real-time optimization of the HCCI combustion phasing by varying the trajectories. The simulation results show that the combustion phasing can be adjusted quickly as desired, which further demonstrates the effectiveness of the piston trajectory-based combustion control.